Methods of forming steel sheets with enhanced flatness

ABSTRACT

The present disclosure provides a method for preparing a steel alloy sheet to enhance flatness. The method includes, inter alia, heating a steel alloy material to a first temperature that is greater than a full-austenitization point for the steel alloy material; holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; air quenching the precursor steel sheet to a second temperature that is less than the first temperature and greater than martensitic transformation starting temperature for the steel alloy material; and cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet. The steel alloy material includes greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority of Chinese Application No. 202210588663.4 filed May 27, 2022. The entire disclosure of the above application is incorporated herein by reference.

INTRODUCTION

This section provides background information related to the present disclosure which is not necessarily prior art.

Roll-forming includes continuous bending (for example, using a plurality of rollers) of relatively long strips of sheet metal, typically coiled steel sheets, into desired cross sections. In particular, roll forming is well suited for preparing constant-profile parts with long lengths and in large quantities. Flatness of the sheet metal as used during roll-forming ensures continuous flow between the rollers, and also, reduces tool wear. Often direct quenching methods using fast cooling mediums (like water) are used during the formation of steel coils. These fast cooling methods often result in thermal distortions, and as result undesirable waviness, in as-formed steel sheets that define the steel coil. Accordingly, it would be desirable to develop steel alloys and methods of producing steel sheets and coils that eliminates or minimizes undesired thermal distortions, and consequently, improve roll-forming processes.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

The present disclosure provides high-strength, high-ductility, high-bendability steel alloys, and also, steel sheets including the steel alloys and having improved flatness. The present disclosure also provides method for heat treatment of the steel sheets, including a direct quenching method, a quenching and partitioning/tempering method, and an austempering method.

In various aspects, the present disclosure provides a method for preparing a steel alloy sheet to enhance flatness. The method may include heating a steel alloy material to a first temperature that is greater than a full-austenitization point for the steel alloy material; holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; air quenching the precursor steel sheet to a second temperature that is less than the first temperature and greater than martensitic transformation starting temperature for the steel alloy material; and cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet. The room temperature may be greater than or equal to about 15° C. to less than or equal to about 25° C.

In one aspect, the first temperature may be greater than or equal to about 800° C. to less than or equal to about 950° C. The second temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C. The cooling rate during the air quenching may be greater than or equal to about 2° C./s to less than or equal to about 15° C./s.

In one aspect, the air quenching may be a first air quenching step, and the method may further include a second air quenching step. The second air quenching step may include air quenching the precursor steel sheet to a third temperature less than the second temperature.

In one aspect, the second air quenching step may be a continuation of the first air quenching step.

In one aspect, the cooling rate during the second air quenching step may be greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s. The third temperature may be less than or equal to about 400° C.

In one aspect, the steel alloy sheet may have a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1600 MPa, and a total elongation greater than or equal to about 3%. The steel alloy sheet may have a microstructure that includes greater than or equal to about 80 vol. % to less than or equal to about 99 vol. % of martensite phase; greater than or equal to about 1 vol. % to less than or equal to about 10 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.

In one aspect, the method may include holding the precursor steel sheet at the third temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

In one aspect, the method may further include heating the precursor steel sheet from the third temperature to a fourth temperature that is less than the first temperature.

In one aspect, the fourth temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C.

In one aspect, the method may further include holding the precursor steel sheet at the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

In one aspect, the steel alloy sheet may have a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1500 MPa, a total elongation greater than or equal to about 7%, and a bending angle greater than or equal to about 50 degrees. The steel alloy sheet may have a microstructure that includes greater than or equal to about 50 vol. % to less than or equal to about 95 vol. % of martensite constituents; greater than or equal to about 5 vol. % to less than or equal to about 17 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 25 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.

In one aspect, the method may further include holding the precursor steel sheet at the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

In one aspect, the steel alloy sheet may have a yield strength greater than or equal to about 1100 MPa, an ultimate tensile strength greater than or equal to about 1550 MPa, a total elongation greater than or equal to about 7%, and bending angle greater than or equal to about 50 degrees. The steel alloy may have a microstructure that includes greater than or equal to about 30 vol. % to less than or equal to about 97 vol. % of martensite constituents; greater than or equal to about 3 vol. % to less than or equal to about 15 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 45 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.

In one aspect, the steel alloy material may include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese; greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, where a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; and a balance of iron.

In one aspect, the steel alloy material may further include greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; and greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium.

In various aspects, the present disclosure provides a method for preparing a steel alloy sheet to enhance flatness. The method may include heating a steel alloy material to a first temperature; holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; a first air quenching of the precursor steel sheet from the first temperature to a second temperature at a first cooling rate greater than or equal to about 2° C./s to less than or equal to about 15° C./s; a second air quenching of the precursor steel sheet from the second temperature to a third temperature at a second cooling rate greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s; and cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet. The first temperature may be greater than or equal to about 800° C. to less than or equal to about 950° C. The second temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C. The third temperature may be less than or equal to about 400° C. The room temperature may be greater than or equal to about 15° C. to less than or equal to about 25° C.

In one aspect, the method may further include holding the precursor steel sheet at the third temperature for a holding period greater than or equal to about 1 second to less than or equal to about 10,000 seconds. The method may also include after the holding period heating the precursor steel sheet from the third temperature to a fourth temperature, and holding the precursor steel sheet at the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds. The fourth temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C.

In one aspect, the steel alloy material may include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese; greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium; and a balance of iron.

In various aspects, the present disclosure provides a method for preparing a steel alloy sheet to enhance flatness. The method may include heating a steel alloy material to a first temperature; holding the steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; a first air quenching of the precursor steel sheet from the first temperature to a second temperature at a first cooling rate greater than or equal to about 2° C./s to less than or equal to about 15° C./s; holding the precursor steel sheet at the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds; and cooling the precursor steel sheet from the second temperature to room temperature to prepare the steel alloy sheet. The first temperature may be first temperature greater than or equal to about 800 ° C. to less than or equal to about 950° C. The second temperature second temperature greater than or equal to about 300° C. to less than or equal to about 500° C. The room temperature may be greater than or equal to about 15° C. to less than or equal to about 25° C.

In one aspect, the steel alloy material may include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese; greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, where a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium; and a balance of iron.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 is a graphical representation demonstrating a direct quenching process for forming steel sheets having improved flatnesses in accordance with various aspects of the present disclosure;

FIG. 2 is a graphical representation demonstrating a quenching and partitioning/tempering process for forming steel sheets with improved flatnesses in accordance with various aspects of the present disclosure;

FIG. 3 is a graphical representation demonstrating an austempering process for forming steel sheets with improved flatnesses in accordance with various aspects of the present disclosure;

FIG. 4 is a graphical representation demonstrating tensile strength and ductility of an example steel sheet prepared using a direct quenching process in accordance with various aspects of the present disclosure;

FIG. 5 is a graphical representation demonstrating tensile strength and ductility of an example steel sheet prepared using a quenching and partitioning/tempering process in accordance with various aspects of the present disclosure; and

FIG. 6 is a graphical representation demonstrating tensile strength and ductility of an example steel sheet prepared using an austempering process in accordance with various aspects of the present disclosure.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, elements, compositions, steps, integers, operations, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Although the open-ended term “comprising,” is to be understood as a non-restrictive term used to describe and claim various embodiments set forth herein, in certain aspects, the term may alternatively be understood to instead be a more limiting and restrictive term, such as “consisting of” or “consisting essentially of.” Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and/or process steps, the present disclosure also specifically includes embodiments consisting of, or consisting essentially of, such recited compositions, materials, components, elements, features, integers, operations, and/or process steps. In the case of “consisting of,” the alternative embodiment excludes any additional compositions, materials, components, elements, features, integers, operations, and/or process steps, while in the case of “consisting essentially of,” any additional compositions, materials, components, elements, features, integers, operations, and/or process steps that materially affect the basic and novel characteristics are excluded from such an embodiment, but any compositions, materials, components, elements, features, integers, operations, and/or process steps that do not materially affect the basic and novel characteristics can be included in the embodiment.

Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated.

When a component, element, or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected, or coupled to the other component, element, or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer, or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer, or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,” “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates both exactly or precisely the stated numerical value, and also, that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. For example, “about” may comprise a variation of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in certain aspects, optionally less than or equal to 0.1%.

In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges.

Example embodiments will now be described more fully with reference to the accompanying drawings.

In various aspects, the present disclosure provides high-strength, high-ductility, high-bendability steel alloys, and also, steel sheets including the steel alloys and having improved flatness. The steel sheets may be used to form components or articles using, for example, roll forming processes. The steel sheets may be used to form components or articles of an automobile or other vehicles (e.g., motorcycles, boats, tractors, buses, motorcycles, mobile homes, campers, and tanks), but they may also be used in a variety of other industries and applications, including aerospace components, consumer goods, devices, buildings (e.g., houses, offices, sheds, warehouses), office equipment and furniture, and industrial equipment machinery, agricultural or farm equipment, or heavy machinery, by way of non-limiting example. Non-limiting examples of automotive components or articles include hoods, pillars (e.g., A-pillars, hinge pillars, B-pillars, C-pillars, and the like), panels, including structural panels, door panels, and door components, interior floors, floor pans, roofs, exterior surfaces, underbody shields, wheels, control arms and other suspension, crush cans, bumpers, structural rails and frames, cross car beams, undercarriage or drive train components, and the like.

In accordance with various aspects of the present disclosure, the steel alloys may include carbon (C), chromium (Cr), silicon (Si), and iron (Fe). In certain variations, the steel alloys may also include manganese (Mn) and/or aluminum (Al). In still further variations, the steel alloys may include vanadium (V), niobium (Nb), and/or titanium (Ti).

In certain variations, the steel alloys may include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon. For example, the steel alloys may include greater than or equal to about 0.05 wt. %, optionally greater than or equal to about 0.1 wt. %, optionally greater than or equal to about 0.15 wt. %, optionally greater than or equal to about 0.2 wt. %, optionally greater than or equal to about 0.25 wt. %, optionally greater than or equal to about 0.3 wt. %, optionally greater than or equal to about 0.35 wt. %, and in certain aspects, optionally greater than or equal to about 0.4 wt. %, of carbon. The steel alloys may include less than or equal to about 0.45 wt. %, optionally less than or equal to about 0.4 wt. %, optionally less than or equal to about 0.35 wt. %, optionally less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.25 wt. %, optionally less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.15 wt. %, and in certain aspects, optionally less than or equal to about 0.1 wt. %, of carbon.

In certain variations, the steel alloys may include greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium. For example, the steel alloys may include greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.5 wt. %, optionally greater than or equal to about 2 wt. %, optionally greater than or equal to about 2.5 wt. %, optionally greater than or equal to about 3 wt. %, optionally greater than or equal to about 3.5 wt. %, optionally greater than or equal to about 4 wt. %, optionally greater than or equal to about 4.5 wt. %, optionally greater than or equal to about 5 wt. %, and in certain aspects, optionally greater than or equal to about 5.5 wt. %, of chromium. The steel alloys may include less than or equal to about 6 wt. %, optionally less than or equal to about 5.5 wt. %, optionally less than or equal to about 5 wt. %, optionally less than or equal to about 4.5 wt. %, optionally less than or equal to about 4 wt. %, optionally less than or equal to about 3.5 wt. %, optionally less than or equal to about 3 wt. %, optionally less than or equal to about 2.5 wt. %, optionally less than or equal to about 2 wt. %, optionally less than or equal to about 1.5 wt. %, and in certain aspects, optionally less than or equal to about 1 wt. %, of chromium. The presence of chromium reduces the cooling rates and allows high hardenability to be obtained during air quenching, which as detailed below, includes lower and more uniform cooling rates as compared to common direct quenching methods using fast cooling mediums, like water.

In certain variations, the steel alloys may include greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon. For example, the steel alloys may include greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 0.75 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.25 wt. %, optionally greater than or equal to about 1.50 wt. %, optionally greater than or equal to about 1.75 wt. %, optionally greater than or equal to about 2 wt. %, and in certain aspects, optionally greater than or equal to about 2.25 wt. %, of silicon. The steel alloys may include less than or equal to about 2.5 wt. %, optionally less than or equal to about 2.25 wt. %, optionally less than or equal to about 2 wt. %, optionally less than or equal to about 1.75 wt. %, optionally less than or equal to about 1.5 wt. %, optionally less than or equal to about 1.25 wt. %, optionally less than or equal to about 1 wt. %, and in certain aspects, optionally less than or equal to about 0.75 wt. %, of silicon.

In certain variations, the steel alloys may include greater than or equal to about 0 wt. % to less than or equal to about 4.5 wt. % of manganese. For example, the steel alloys may include greater than or equal to about 0 wt. %, optionally greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.5 wt. %, optionally greater than or equal to about 2 wt. %, optionally greater than or equal to about 2.5 wt. %, optionally greater than or equal to about 3 wt. %, optionally greater than or equal to about 3.5 wt. %, and in certain aspects, optionally greater than or equal to about 4 wt. %, of manganese. The steel alloys may include less than or equal to about 4.5 wt. %, optionally less than or equal to about 4 wt. %, optionally less than or equal to about 3.5 wt. %, optionally less than or equal to about 3 wt. %, optionally less than or equal to about 2.5 wt. %, optionally less than or equal to about 2 wt. %, optionally less than or equal to about 1.5 wt. %, optionally less than or equal to about 1 wt. %, and in certain aspects, optionally less than or equal to about 0.5 wt. %, of manganese.

In certain variation, the steel alloys may include greater than or equal to about 0 wt. % to less than or equal to about 2 wt. % of aluminum. For example, the steel alloys may include greater than or equal to about 0 wt. %, optionally greater than or equal to about 0.25 wt. %, optionally greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 0.75 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.25 wt. %, optionally greater than or equal to about 1.5 wt. %, and in certain aspects, optionally greater than or equal to about 1.75 wt. %, of aluminum. The steel alloys may include less than or equal to about 2 wt. %, optionally less than or equal to about 1.75 wt. %, optionally less than or equal to about 1.5 wt. %, optionally less than or equal to about 1 wt. %, optionally less than or equal to about 0.75 wt. %, optionally less than or equal to about 0.5 wt. %, and in certain aspects, optionally less than or equal to about 0.25 wt. %, or aluminum. In certain variations, a chromium to aluminum ratio may be greater than or equal to about 1.7, so as to aid in the hardenability during quenching. In still further variations, the sum of aluminum and silicon may be greater than or equal to about 0.7 wt. %, so as to stabilize retained austenite at room temperature following the heat treatment processes.

In certain variations, the steel alloys may include greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium. For example, the steel alloys may include greater than or equal to 0 wt. %, optionally greater than or equal to 0.05 wt. %, optionally greater than or equal to 0.1 wt. %, optionally greater than or equal to 0.15 wt. %, optionally greater than or equal to 0.2 wt. %, optionally greater than or equal to 0.25 wt. %, optionally greater than or equal to 0.3 wt. %, optionally greater than or equal to 0.35 wt. %, optionally greater than or equal to 0.4 wt. %, and in certain aspects, optionally greater than or equal to 0.45 wt. %, of vanadium. The steel alloys may include less than or equal to about 0.5 wt. %, optionally less than or equal to about 0.45 wt. %, optionally less than or equal to about 0.4 wt. %, optionally less than or equal to about 0.35 wt. %, optionally less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.25 wt. %, optionally less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.15 wt. %, optionally less than or equal to about 0.1 wt. %, and in certain aspects, optionally less than or equal to about 0.05 wt. %, of vanadium.

In certain variations, the steel alloys may include greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium. For example, the steel alloys may include greater than or equal to about 0 wt. %, optionally greater than or equal to about 0.02 wt. %, optionally greater than or equal to about 0.04 wt. %, optionally greater than or equal to about 0.06 wt. %, optionally greater than or equal to about 0.08 wt. %, optionally greater than or equal to about 0.1 wt. %, optionally greater than or equal to about 0.12 wt. %, optionally greater than or equal to about 0.14 wt. %, optionally greater than or equal to about 0.16 wt. %, and in certain aspects, optionally greater than or equal to about 0.18 wt. %, of niobium. The steel alloys may include less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.18 wt. %, optionally less than or equal to about 0.16 wt. %, optionally less than or equal to about 0.14 wt. %, optionally less than or equal to about 0.12 wt. %, optionally less than or equal to about 0.1 wt. %, optionally less than or equal to about 0.08 wt. %, optionally less than or equal to about 0.06 wt. %, optionally less than or equal to about 0.04 wt. %, and in certain aspects, optionally less than or equal to about 0.02 wt. %, of niobium.

In certain variations, the steel alloys may include greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium. For example, the steel alloys may include greater than or equal to 0 wt. %, optionally greater than or equal to 0.02 wt. %, optionally greater than or equal to about 0.04 wt. %, optionally greater than or equal to about 0.06 wt. %, optionally greater than or equal to about 0.08 wt. %, optionally greater than or equal to about 0.1 wt. %, optionally greater than or equal to about 0.12 wt. %, optionally greater than or equal to about 0.14 wt. %, optionally greater than or equal to about 0.16 wt. %, optionally greater than or equal to about 0.18 wt. %, optionally greater than or equal to about 0.2 wt. %, optionally greater than or equal to about 0.22 wt. %, optionally greater than or equal to about 0.24 wt. %, optionally greater than or equal to about 0.26 wt. %, and in certain aspects, optionally greater than or equal to about 0.28 wt. %, of titanium. The steel alloys may include less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.28 wt. %, optionally less than or equal to about 0.26 wt. %, optionally less than or equal to about 0.24 wt. %, optionally less than or equal to about 0.22 wt. %, optionally less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.18 wt. %, optionally less than or equal to about 0.16 wt. %, optionally less than or equal to about 0.14 wt. %, optionally less than or equal to about 0.12 wt. %, optionally less than or equal to about 0.1 wt. %, optionally less than or equal to about 0.08 wt. %, optionally less than or equal to about 0.06 wt. %, optionally less than or equal to about 0.04 wt. %, and in certain aspects, optionally less than or equal to about 0.02 wt. %, of titanium.

In each variation, the steel alloys include a balance of iron. For example, the steel alloys may include greater than or equal to about 80 wt. %, optionally greater than or equal to about 81 wt. %, optionally greater than or equal to about 82 wt. %, optionally greater than or equal to about 83 wt. %, optionally greater than or equal to about 84 wt. %, optionally greater than or equal to about 85 wt. %, optionally greater than or equal to about 86 wt. %, optionally greater than or equal to about 87 wt. %, optionally greater than or equal to about 88 wt. %, optionally greater than or equal to about 89 wt. %, optionally greater than or equal to about 90 wt. %, optionally greater than or equal to about 91 wt. %, optionally greater than or equal to about 92 wt. %, optionally greater than or equal to about 93 wt. %, optionally greater than or equal to about 94 wt. %, optionally greater than or equal to about 95 wt. %, optionally greater than or equal to about 96 wt. %, optionally greater than or equal to about 97 wt. %, and in certain aspects, optionally greater than or equal to about 98 wt. %, or iron.

As mentioned above, steel alloys in accordance with various aspects of the present disclosure may provide steel sheets having improved flatness. In various aspects, the present disclosure provides heat treatment methods or processes for the steel sheets. After the heat treatment, the steel sheets may be coiled and moved, for example, to a roll forming mill for forming. The example methods include various air quenching or cooling steps, and as such, avoid thermal distortions that commonly result from the use of fast cooling medium like water during direct quenching (for example, at steps 130, 230, 330 as detailed below). Air quenching includes natural cooling in the atmosphere, or in certain instances, forcing air or gas over the steel alloy.

The as-prepared steel sheets may have high strength, and also, high ductility and bendability. For example, the microstructure of the example steel sheet 400 may include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength and the retained austenite is associated with high ductility and bendability. In certain variations, the as-prepared steel sheets may have include greater than or equal to about 30 vol. % to less than or equal to about 99 vol. % of martensite phase; greater than or equal to about 1 vol. % to less than or equal to about 17 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 45 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase. Also, the as-prepared steel sheets may yield strengths (YS) greater than or equal to about 1100 MPa, and in certain aspects, optionally greater than or equal to about 1100 MPa to less than or equal to about 1500 MPa. The as-prepared steel sheet may have an ultimate tensile strengths (UTS) greater than or equal to about 1500 MPa. The as-prepared steel sheets may have total elongations (TEL) greater than or equal to about 3%, and in certain aspects, optionally greater than or equal to about 6%. The as-prepared steel sheets may have bending angles greater than or equal to about 45 degrees, and in certain aspects, optionally greater than or equal to about 50 degrees.

FIG. 1 is a graphical illustration that summarizes an example direct quenching process 100 for forming steel sheets with high flatness, where the x-axis 102 represents time in seconds, and the y-axis 104 represents temperature in degrees Celsius. As illustrated, the method 100 includes heating 110 a precursor sheet including a steel alloy to a first temperature. Although not illustrated, the skilled artisan will understand that in certain variations, the precursor sheet is unrolled from a steel coil.

The first temperature is above an austenitization point for the steel alloy (which is represented by line 112). For example, in certain variations, the first temperature may be greater than or equal to about 800° C. to less than or equal to about 950° C., optionally greater than or equal to about 850° C. to less than or equal to about 950° C., and in certain aspects, optionally about 930° C. The precursor sheet may be heated to the first temperature at a rate greater than or equal to about 0.1° C.·s⁻¹ to less than or equal to about 100° C.·s⁻¹. The method 100 includes holding or soaking 120 the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 200 seconds to less than or equal to about 500 seconds.

The holding 120 may be followed by a first air quenching 130. For example, the precursor sheet may be cooled to a second temperature, which is less than the first temperature. The second temperature may be between about 500° C. (which is represented by line 132) and a martensitic transformation starting temperature (which is represented by line 134). For example, the second temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C., and in certain aspects, optionally greater than or equal to about 400° C. to less than or equal to about 500° C. The cooling rate (which is method dependent) should be higher than or closer to the critical cooling rate necessary to obtained martensite transformation for high strength. For example, in certain variations, the cooling rate may be greater than or equal to about 2° C./s to less than or equal to about 15° C./s may apply. The first air quenching 130 may be a natural air cooling process and/or a forced air/gas cooling process.

The first air quenching 130 may be followed by a second air quenching 140. For example, the precursor sheet may be cooled to a third temperature, which is less than the second temperature. The third temperature is less than the martensitic transformation starting temperature. For example, the third temperature may be less than or equal to about 400° C., and in certain aspects, optionally less than or equal to about 300° C. In certain variations, the third temperature may be less than or equal to about 400° C., and in certain aspects, optionally less than or equal to about 300° C. The third temperature may be greater than or equal to room temperature (e.g., greater than or equal to about 15° C. to less than or equal to about 25° C.). A cooling rate greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s may apply.

Following the second air quenching 140, the method 100 may further include cooling 150 the precursor sheet to room temperature (e.g., greater than or equal to about 15° C. to less than or equal to about 25° C.) to obtain a steel sheet with improved flatness. For example, the steel sheet may be air cooled to room temperature. As the skilled artisan will recognize, the cooling rates are slower at lower temperature because temperature differences between the steel sheet and the atmosphere is lower, accounting for the differences between the first air quenching 130, the second air quenching 140, and the cooling 150.

As noted, the as-prepared steel sheet has improved flatness. For example, when the as-prepared steel sheet has a thickness greater than or equal to about 0.8 mm to less than or equal to about 1.3 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 7 mm, and in certain aspects, optionally less than or equal to about 6 mm. When the as-prepared steel sheet has a thickness greater than or equal to about 1.3 mm to less than or equal to about 1.8 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 6 mm, and in certain aspects, optionally less than or equal to about 5 mm. When the as-prepared steel sheet has a thickness greater than or equal to about 1.8 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 5 mm, and in certain aspects, optionally less than or equal to about 4 mm.

Although not illustrated, in certain variations, the method 100 may further include coiling the steel sheet having improved flatness and moving the formed coil to a roll forming mill for forming.

FIG. 2 is a graphical illustration that summarizes an example quenching and partitioning/tempering process 200 for forming steel sheets with high flatness, where the x-axis 202 represents time in seconds, and the y-axis 204 represents temperature in degrees Celsius. As illustrated, the method 200 includes heating 210 a precursor sheet including a steel alloy to a first temperature. Although not illustrated, the skilled artisan will understand that in certain variations, the precursor sheet is unrolled from a steel coil.

The first temperature is above an austenitization point for the steel alloy (which is represented by line 212). For example, in certain variations, the first temperature may be greater than or equal to about 800° C. to less than or equal to about 950° C., optionally greater than or equal to about 850° C. to less than or equal to about 950° C., and in certain aspects, optionally about 930° C. The precursor sheet may be heated to the first temperature at a rate greater than or equal to about 0.1° C.·s⁻¹ to less than or equal to about 100° C.·s⁻¹. The method 200 includes holding or soaking 220 the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, greater than or equal to about 200 seconds to less than or equal to about 500 seconds, and in certain aspects, optionally about 340 seconds.

The holding 220 may be followed by a first air quenching 230. For example, the precursor sheet may be cooled to a second temperature that is less than the first temperature. The second temperature may be between about 500° C. (which is represented by line 232) and a martensitic transformation starting temperature (which is represented by line 234). For example, the second temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C., and in certain aspects, optionally greater than or equal to about 400° C. to less than or equal to about 500° C. The cooling rate (which is method dependent) should be higher than or closer to the critical cooling rate necessary to obtained martensite transformation for high strength. For example, in certain variations, the cooling rate may be greater than or equal to about 2° C./s to less than or equal to about 15° C./s may apply. The first air quenching 230 may be a natural air cooling process and/or a forced air/gas cooling process.

The first air quenching 230 may be follow by a second air quenching 240. For example, the precursor sheet may be cooled to a third temperature that is less than the second temperature. Like the first air quenching 230, the second air quenching 240 may be a natural air cooling process and/or a forced air/gas cooling process. A cooling rate greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s may apply. The third temperature is less than the martensitic transformation starting temperature. For example, the third temperature may be less than or equal to about 400° C., and in certain aspects, optionally less than or equal to about 300° C. In certain variations, the third temperature may be less than or equal to about 400° C., and in certain aspects, optionally less than or equal to about 300° C. The third temperature may be greater than or equal to room temperature (e.g., greater than or equal to about 15° C. to less than or equal to about 25° C.). The method 200 may further include holding 250 the third temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 20 second to less than or equal to about 100 seconds.

After the second air quenching 240 and the holding 250, the method 200 may include heating 260 the precursor sheet to a fourth temperature that is greater than the third temperature. Like the second temperature, the fourth temperature may be between about 500° C. (which is represented by line 232) and a martensitic transformation starting temperature (which is represented by line 234). For example, the fourth temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C., and in certain aspects, optionally greater than or equal to about 400° C. to less than or equal to about 500° C. The fourth temperature may be the same or different from the second temperature. The precursor sheet may be heated to the fourth temperature at a rate greater than or equal to about 0.1° C.·s⁻¹ to less than or equal to about 100° C.·s⁻¹. The method 200 may include holding 270 the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 20 second to less than or equal to about 100 seconds. Following the third holding period, the method 200 may further include cooling 280 the precursor sheet to room temperature (e.g., greater than or equal to about 15° C. to less than or equal to about 25° C.) to obtain a steel sheet having improved flatness. For example, the steel sheet may be air cooled to room temperature.

As noted, the as-prepared steel sheet has improved flatness. For example, when the as-prepared steel sheet has a thickness greater than or equal to about 0.8 mm to less than or equal to about 1.3 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 7 mm, and in certain aspects, optionally less than or equal to about 6 mm. When the as-prepared steel sheet has a thickness greater than or equal to about 1.3 mm to less than or equal to about 1.8 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 6 mm, and in certain aspects, optionally less than or equal to about 5 mm. When the as-prepared steel sheet has a thickness greater than or equal to about 1.8 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 5 mm, and in certain aspects, optionally less than or equal to about 4 mm.

Although not illustrated, in certain variations, the method 200 may further include coiling the steel sheet having improved flatness and moving the formed coil to a roll forming mill for forming.

FIG. 3 is a graphical illustration that summarizes an example austempering process 300 for forming steel sheets with high flatness, where the x-axis 302 represents time in seconds, and the y-axis 304 represents temperature in degrees Celsius. As illustrated, the method 300 includes heating 310 a precursor sheet including a steel alloy to a first temperature. Although not illustrated, the skilled artisan will understand that in certain variations, the precursor sheet is unrolled from a steel coil

The first temperature is above an austenitization point for the steel alloy (which is represented by line 112). For example, in certain variations, the first temperature may be greater than or equal to about 800° C. to less than or equal to about 950° C., optionally greater than or equal to about 850° C. to less than or equal to about 950° C., and in certain aspects, optionally about 930° C. The precursor sheet may be heated to the first temperature at a rate greater than or equal to about 0.1° C.·s⁻¹ to less than or equal to about 100° C.·s⁻¹. The method 100 includes holding or soaking 120 the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, optionally greater than or equal to about 200 seconds to less than or equal to about 500 seconds, and in certain aspects, optionally about 340 seconds.

The first holding step 120 may be followed by a first air quenching 330. For example, the precursor sheet may be cooled to a second temperature, which is less than the first temperature. The second temperature may be between about 500° C. (which is represented by line 132) and a martensitic transformation starting temperature (which is represented by line 134). For example, the second temperature may be greater than or equal to about 300° C. to less than or equal to about 500° C., and in certain aspects, optionally greater than or equal to about 400° C. to less than or equal to about 500° C. The cooling rate (which is method dependent) should be higher than or closer to the critical cooling rate necessary to obtained martensite transformation for high strength. For example, in certain variations, the cooling rate may be greater than or equal to about 2° C./s to less than or equal to about 15° C./s may apply. The first air quenching 330 may be a natural air cooling process and/or a forced air/gas cooling process.

The method may further include holding 340 the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 20 seconds to less than or equal to about 100 seconds. Following the second holding period, the method 300 may further include cooling 350 the precursor sheet to room temperature (e.g., greater than or equal to about 15° C. to less than or equal to about 25° C.) to obtain a steel sheet having improved flatness. For example, the steel sheet may be air cooled to room temperature.

As noted, the as-prepared steel sheet has improved flatness. For example, when the as-prepared steel sheet has a thickness greater than or equal to about 0.8 mm to less than or equal to about 1.3 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 7 mm, and in certain aspects, optionally less than or equal to about 6 mm. When the as-prepared steel sheet has a thickness greater than or equal to about 1.3 mm to less than or equal to about 1.8 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 6 mm, and in certain aspects, optionally less than or equal to about 5 mm. When the as-prepared steel sheet has a thickness greater than or equal to about 1.8 mm, a maximum distance between a normal plane and various peaks in the sheet (i.e., height) may be less than or equal to about 5 mm, and in certain aspects, optionally less than or equal to about 4 mm.

Although not illustrated, in certain variations, the method 300 may further include coiling the steel sheet having improved flatness and moving the formed coil to a roll forming mill for forming.

Certain features of the current technology are further illustrated in the following non-limiting examples.

Example 1

Example steel sheets may be prepared in accordance with various aspects of the present disclosure. For example, an example steel sheet 400 may be prepared from a steel alloy using a direct quenching process, like the direct quenching process illustrated in FIG. 1 . FIG. 4 is a graphical illustration demonstrating the tensile strength and ductility of the example steel sheet 400, where the x-axis 402 represents tensile strain (mm/mm), and the y-axis 404 represents tensile stress (MPa).

The example steel sheet 400 may have high strength, and also, high ductility and bendability. For example, the microstructure of the example steel sheet 400 may include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength and the retained austenite is associated with high ductility and bendability. In certain variations, the example steel sheet 400 may include greater than or equal to about 80 vol. % to less than or equal to about 99 vol. % of martensite phase; greater than or equal to about 1 vol. % to less than or equal to about 10 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.

The example steel sheet 400 may have a yield strength (YS) greater than or equal to about 1150 MPa, with a standard deviation of about 3 MPa.

The example steel sheet 400 may have an ultimate tensile strength (UTS) greater than or equal to about 1600 MPa, with a standard deviation of about 3 MPa.

The example steel sheet 400 may have a total elongation (TEL) greater than or equal to about 3%, with a standard deviation of about 3%.

Example 2

Example steel sheets may be prepared in accordance with various aspects of the present disclosure. For example, an example steel sheet 500 may be prepared from a steel alloy using a quenching and partitioning/tempering process, like the quenching and partitioning/tempering process illustrated in FIG. 2 . FIG. 5 is a graphical illustration demonstrating the tensile strength and ductility of the example steel sheet 500, where the x-axis 502 represents tensile strain (mm/mm), and the y-axis 504 represents tensile stress (MPa).

The example steel sheet 500 may have high strength, and also, high ductility and bendability. For example, the microstructure of the example steel sheet 500 may include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength and the retained austenite is associated with high ductility and bendability. In certain variations, the example steel sheet 500 may include greater than or equal to about 50 vol. % to less than or equal to about 95 vol. % of martensite constituents; greater than or equal to about 5 vol. % to less than or equal to about 17 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 25 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.

The example steel sheet 500 may have a yield strength (YS) greater than or equal to about 1150 MPa, with a standard deviation of about 3 MPa.

The example steel sheet 500 may have an ultimate tensile strength (UTS) greater than or equal to about 1500 MPa, with a standard deviation of about 3 MPa.

The example steel sheet 500 may have a total elongation (TEL) greater than or equal to about 7%, with a standard deviation of about 3%.

The example steel sheet 500 may a bendability (i.e., bending angled), measured, for example, using standard VDA 238-100, greater than or equal to about 50 degrees, with a standard deviation of about 3 degrees.

Example 3

Example steel sheets may be prepared in accordance with various aspects of the present disclosure. For example, an example steel sheet 600 may be prepared from a steel alloy using an austempering process, like the austempering process illustrated in FIG. 3 . FIG. 6 is a graphical illustration demonstrating the tensile strength and ductility of the example steel sheet 600, where the x-axis 602 represents tensile strain (mm/mm), and the y-axis 604 represents tensile stress (MPa).

The example steel sheet 600 may have high strength, and also, high ductility and bendability. For example, the microstructure of the example steel sheet 600 may include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength and the retained austenite is associated with high ductility and bendability. In certain variations, the example steel sheet 600 may include greater than or equal to about 30 vol. % to less than or equal to about 97 vol. % of martensite constituents; greater than or equal to about 3 vol. % to less than or equal to about 15 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 45 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.

The example steel sheet 600 may have a yield strength (YS) greater than or equal to about 1100 MPa, with a standard deviation of about 3 MPa.

The example steel sheet 600 may have an ultimate tensile strength (UTS) greater than or equal to about 1550 MPa, with a standard deviation of about 3 MPa.

The example steel sheet 600 may have a total elongation (TEL) greater than or equal to about 7%, with a standard deviation of about 3%.

The example steel sheet 600 may bendability (i.e., bending angled), measured, for example, using standard VDA 238-100, greater than or equal to about 50 degrees, with a standard deviation of about 3 degrees.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

What is claimed is:
 1. A method for preparing a steel alloy sheet to enhance flatness, the method comprising: heating a steel alloy material to a first temperature that is greater than a full-austenitization point for the steel alloy material; holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; air quenching the precursor steel sheet to a second temperature that is less than the first temperature and greater than martensitic transformation starting temperature for the steel alloy material; and cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15° C. to less than or equal to about 25° C.
 2. The method of claim 1, wherein the first temperature is greater than or equal to about 800° C. to less than or equal to about 950° C., the second temperature is greater than or equal to about 300° C. to less than or equal to about 500° C., and the cooling rate during the air quenching is greater than or equal to about 2° C./s to less than or equal to about 15° C./s.
 3. The method of claim 1, wherein the air quenching is a first air quenching step, and the method further comprises a second air quenching step, the second air quenching step comprising air quenching the precursor steel sheet to a third temperature less than the second temperature.
 4. The method of claim 3, wherein the second air quenching step is a continuation of the first air quenching step.
 5. The method of claim 3, wherein the cooling rate during the second air quenching step is greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s, and the third temperature is less than or equal to about 400° C.
 6. The method of claim 3, wherein the steel alloy sheet has a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1600 MPa, and a total elongation greater than or equal to about 3%, and wherein the steel alloy sheet has a microstructure that comprises greater than or equal to about 80 vol. % to less than or equal to about 99 vol. % of martensite phase; greater than or equal to about 1 vol. % to less than or equal to about 10 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.
 7. The method of claim 3, wherein the method comprises holding the precursor steel sheet at the third temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.
 8. The method of claim 6, wherein the method further comprises heating the precursor steel sheet from the third temperature to a fourth temperature that is less than the first temperature.
 9. The method of claim 8, wherein the fourth temperature is greater than or equal to about 300° C. to less than or equal to about 500° C.
 10. The method of claim 8, wherein the method further comprises holding the precursor steel sheet at the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.
 11. The method of claim 10, wherein the steel alloy sheet has a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1500 MPa, a total elongation greater than or equal to about 7%, and a bending angle greater than or equal to about 50 degrees, and wherein the steel alloy sheet has a microstructure that comprises greater than or equal to about 50 vol. % to less than or equal to about 95 vol. % of martensite constituents; greater than or equal to about 5 vol. % to less than or equal to about 17 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 25 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.
 12. The method of claim 1, wherein the method further comprises holding the precursor steel sheet at the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.
 13. The method of claim 12, wherein the steel alloy sheet has a yield strength greater than or equal to about 1100 MPa, an ultimate tensile strength greater than or equal to about 1550 MPa, a total elongation greater than or equal to about 7%, and bending angle greater than or equal to about 50 degrees, and wherein the steel alloy has a microstructure that comprises greater than or equal to about 30 vol. % to less than or equal to about 97 vol. % of martensite constituents; greater than or equal to about 3 vol. % to less than or equal to about 15 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 45 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.
 14. The method of claim 1, wherein the steel alloy material comprises: greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese; greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; and a balance of iron.
 15. The method of claim 1, wherein the steel alloy material further comprises: greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; and greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium.
 16. A method for preparing a steel alloy sheet to enhance flatness, the method comprising: heating a steel alloy material to a first temperature greater than or equal to about 800° C. to less than or equal to about 950° C.; holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; a first air quenching of the precursor steel sheet from the first temperature to a second temperature greater than or equal to about 300° C. to less than or equal to about 500° C. at a first cooling rate greater than or equal to about 2° C./s to less than or equal to about 15° C./s; a second air quenching of the precursor steel sheet from the second temperature to a third temperature less than or equal to about 400° C. at a second cooling rate greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s; and cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15° C. to less than or equal to about 25° C.
 17. The method of claim 16, wherein the method further comprises holding the precursor steel sheet at the third temperature for a holding period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, after the holding period heating the precursor steel sheet from the third temperature to a fourth temperature, and holding the precursor steel sheet at the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, the fourth temperature being greater than or equal to about 300° C. to less than or equal to about 500° C.
 18. The method of claim 16, wherein the steel alloy material comprises: greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese; greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium; and a balance of iron.
 19. A method for preparing a steel alloy sheet to enhance flatness, the method comprising: heating a steel alloy material to a first temperature greater than or equal to about 800° C. to less than or equal to about 950° C.; holding the steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; a first air quenching of the precursor steel sheet from the first temperature to a second temperature greater than or equal to about 300° C. to less than or equal to about 500° C. at a first cooling rate greater than or equal to about 2° C./s to less than or equal to about 15° C./s; holding the precursor steel sheet at the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds; and cooling the precursor steel sheet from the second temperature to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15° C. to less than or equal to about 25° C.
 20. The method of claim 19, wherein the steel alloy material comprises: greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese; greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium; and a balance of iron. 